Method for making thermal interface material

BACKGROUND

00011. Field of the Invention

0002The present invention relates to methods for making thermal interface material, and particularly to a method for making carbon-nanotube-based thermal interface material.

00032. Discussion of Related Art

0004Electronic components such as semiconductor chips are becoming progressively smaller, while at the same time heat dissipation requirements thereof are increasing. Commonly, thermal interface material is utilized between the electronic component and an integrated heat spreader/sink, in order to efficiently dissipate heat generated by the electronic component. However, the performance of the thermal interface material is generally restricted by a heat conduction coefficient thereof. The heat conduction coefficient of typical thermal interface units is now considered to be too low for many contemporary applications.

0005Conventional thermal interface materials are metallic materials. However, the metallic materials disadvantageously tend to have a much higher coefficient of thermal expansion than do semiconductor devices. Mechanical stresses are induced during temperature cycling and will tend to overstress the electronic components, thereby leading to potential failures when metallic materials are used as a thermal interface material.

0006A recently developed thermal interface material is a composite material obtained by combining carbon nanotube arrays with a polymer or a low melting point metallic material. The carbon nanotubes are orderly distributed and provide a heat conduction path therein. Additionally, the carbon nanotubes can protrude from the polymer or low melting point metallic material and contact the electronic components or the integrated heat spreaders. However, the heat conduction coefficient of the polymer is relatively low. As such, the performance of the thermal interface material is restricted by the polymer used. Since metals generally have a higher heat conduction coefficient than polymers, the thermal interface materials using carbon nanotube arrays combined with low melting point metallic materials have attracted lots of attention, due to the potential for improved heat conduction coefficients.

0007Methods for making the thermal interface material using low melting point metallic materials combined with carbon nanotube arrays generally have employed chemical vapor deposition or physical vapor deposition. In these methods, the low melting point metallic materials are heated to a gaseous state and deposited in the interspaces between the carbon nanotubes of the carbon nanotube array. However, although some of these metallic materials have a low melting point, they also have a very high boiling point, e.g., the melting point of indium is 157° C. and the boiling point thereof is 2000° C. Thus, the above-described methods consume lots of energy, and the costs thereof are quite high.

0008What is needed, therefore, is a method for making a carbon-nanotube-based thermal interface material, wherein the low melting point metals just need to be heated to a liquid state.

SUMMARY

0009In one embodiment, a method for making a thermal interface material includes the steps of: (a) providing an array of carbon nanotubes formed on a substrate, the carbon nanotubes having interfaces defined therebetween; (b) providing a transferring device and disposing at least one low melting point metallic material above the array of carbon nanotubes, using the transferring device; and (c) heating the low melting point metallic material and the array of carbon nanotube to a certain temperature to make the at least one low melting point metallic material melt, then flow into the interspaces between the carbon nanotubes, and combine (e.g., mechanically) with the array of carbon nanotubes to acquire/yield a carbon-nanotube-based thermal interface material.

0010Other advantages and novel features of the present method for making the thermal interface material will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0011Many aspects of the present invention of the method for making the thermal interface material can be better understood with reference to the following drawings.

0012<figref idref="DRAWINGS">FIG. 1</figref> is flow chart of a method for making a thermal interface material, according to the present embodiment.

0013<figref idref="DRAWINGS">FIG. 2</figref> is a schematic view of a device for making the thermal interface material, according to the present embodiment.

0014Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one preferred embodiment of the present method for making the thermal interface material, in at least one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

0015Reference will now be made to the drawings to describe, in detail, embodiments of the present method for making the thermal interface material.

0016Referring to <figref idref="DRAWINGS">FIG. 1</figref>, a method for making a thermal interface material includes the steps of: (a) providing an array of carbon nanotubes formed on a substrate, the carbon nanotubes having interfaces defined therebetween; (b) providing a transferring device and disposing at least one low melting point metallic material above the array of carbon nanotubes, using the transferring device; and (c) heating the at least one low melting point metallic material and the array of carbon nanotube to a certain temperature to make the at least one low melting point metallic material melt, then flow into the interspaces between carbon nanotubes of the array of carbon nanotubes, and combine (e.g., mechanically) with the array of carbon nanotubes to acquire a carbon-nanotube-based thermal interface material.

0017In step (a), the array of carbon nanotubes is, beneficially, a super-aligned array of carbon nanotubes. The super-aligned array of carbon nanotubes can be formed by the substeps of: (a1) providing a substantially flat and smooth substrate; (a2) forming a catalyst layer on the substrate; (a3) annealing the substrate with the catalyst at a temperature in the approximate range from 700° C. to 900° C. in air for about 30-90 minutes; (a4) heating the substrate with the catalyst at a temperature in the approximate range from 500° C. to 740° C. in a furnace with a protective gas therein; and (a5) supplying a carbon source gas into the furnace for about 5-30 minutes and growing a super-aligned array of the carbon nanotubes from the substrate.

0018In step (a1), the substrate can, beneficially, be a P-type silicon wafer, an N-type silicon wafer, or a silicon wafer with a film of silicon dioxide thereon. Quite usefully, a 4-inch P-type silicon wafer is used as the substrate. In step (a2), the catalyst can, advantageously, be made of iron (Fe), cobalt (Co), nickel (Ni), or any alloy thereof.

0019In step (a4), the protective gas can, beneficially, be made up of at least one of nitrogen (N₂), ammonia (NH₃), and a noble gas. In step (a5), the carbon source gas can, advantageously, be a hydrocarbon gas, such as ethylene (C₂H₄), methane (CH₄), acetylene (C₂H₂), ethane (C₂H₆), or any combination thereof.

0020The super-aligned array of carbon nanotubes can, opportunely, be in a height of about 1 micron to about 500 microns and includes a plurality of carbon nanotubes parallel to each other and substantially perpendicular to the substrate. The super-aligned array of carbon nanotubes formed under the above conditions is essentially free of impurities, such as carbonaceous or residual catalyst particles. The carbon nanotubes in the super-aligned array are packed together closely by van der Waals attractive force. The array of carbon nanotubes formed on the substrate has a first surface and a second surface. The second surface of the array of carbon nanotubes contacts the substrate.

0021It is to be understood that the array of carbon nanotubes can, alternatively, be formed by other methods known in the art.

0022Referring to <figref idref="DRAWINGS">FIG. 2</figref>, in step (b), the transferring device <b>100</b> includes a transferring plate <b>102</b> and a sealed ring <b>110</b>. The transferring plate <b>102</b> has a cavity <b>106</b> therein that is filled with at least one low melting point metallic material <b>104</b>. Such a cavity <b>106</b> is arranged facing the array of carbon nanotubes. The transferring plate <b>102</b> has a plurality of air holes <b>108</b> to permit communication of the cavity <b>106</b>, filled with the at least one low melting point metallic material <b>104</b>, with an exterior space (e.g., ambient). The sealed ring <b>110</b> is disposed between the transferring plate <b>102</b> and substrate <b>12</b>, thereby forming a space therebetween and the array of carbon nanotubes is disposed in the space. The sealed ring <b>110</b> supports the transferring plate <b>102</b> above the array of carbon nanotubes <b>10</b> and maintains a constant gap <b>112</b> to allow the at least one low melting point metallic material <b>104</b> to flow (e.g., due to gravity and/or surface wetting) into the array of carbon nanotubes <b>10</b>.

0023The melting point of the at least one low melting point metallic material <b>104</b> is, advantageously, less than about 265° C. The melting point of the at least one low melting point metallic material <b>104</b> is, beneficially, low, thereby allowing the low melting point metallic material <b>104</b> to readily change to a liquid state and thus better promote heat dissipation when the temperature of the electronic component is high. The at least one low melting point metallic material <b>104</b> may, for example, include tin, copper, indium, lead, antimony, gold, silver, bismuth, or alloys thereof or mixture thereof, and so on. The at least one low melting point metallic material <b>104</b> can be a single-layer structure or a multi-layer structure. The single-layer structure of the at least one low melting point metallic material <b>104</b> can, e.g., be a tin-lead alloy, indium-tin alloy, tin-silver-copper alloy, gold-silicon alloy, gold-germanium alloy, and so on. The multi-layer structure of the at least one low melting point metallic material <b>104</b> can, for example, be a three-layer structure, e.g., one layer of indium, a second layer of a tin alloy, and the other layer of gold-silicon alloy; a two-layer structure, e.g., one layer of indium and the other layer of a bismuth alloy; or any other appropriate multi-layer structure.

0024In step(c), the temperature must be higher than the melting point of the at least one low melting point metallic material <b>104</b>. The melted at least one low melting point metallic material <b>104</b> flows out from the cavity <b>106</b> into the interspaces between the carbon nanotubes of the array of carbon nanotubes <b>10</b>. After that, a negative pressure is applied to the furnace <b>114</b> to extract air from between the carbon nanotubes and the melted low melting point metallic material <b>104</b> to make the array of carbon nanotubes <b>10</b> and the low melting point metallic material <b>104</b> combine sufficiently (i.e., to promote a sufficiently uniform distribution of the low melting point metallic material <b>104</b> relative to the array of carbon nanotubes <b>10</b>). The extracted air is, beneficially, forced out through the air holes <b>108</b> of the transferring plate <b>102</b>. After combining the array of carbon nanotubes <b>10</b> with the low melting point metallic material <b>104</b>, a thermal interface material is formed on the substrate <b>12</b>, and after the furnace <b>114</b> has cooled down, the thermal interface material can be removed from the substrate to be a stand-alone thermal interface material. Alternatively, the thermal interface material can be retained, as a unit, with the substrate <b>12</b> to define a heat spreader and/or a heat sink.

0025The combination of the low melting point metallic material <b>104</b> and the array of carbon nanotubes <b>10</b> provides a thermal interface material with a high heat conduction coefficient. Additionally, the thermal expansion coefficient of the carbon nanotube/metal composite of the resulting thermal interface material more closely matches that of a typical semiconductor material than the thermal expansion coefficient of a typical metal would.

0026In use, the thermal interface material of the present embodiment can be disposed between an electronic component and a heat spreader/heat sink. When a temperature is elevated above the melting point of the low melting point metallic material <b>104</b>, the low melting point metallic material <b>104</b> changes to a liquid, with the sealed ring <b>110</b> serving to generally retain the molten metal. The liquefied low melting point metallic material <b>104</b> is capable of filling the gaps between the electronic component and the heat spreader and, as such, reduces the thermal contact resistance therebetween. Further, due to a high thermal conductivity of the metallic material, the thermal interface material has a higher thermal conductivity than a polymer-carbon-nanotube composite. Additionally, during temperature cycling, mechanical stress of the metallic material can be reduced by the carbon nanotubes dispersed therein. What is more, the method for making the thermal interface material of the present embodiment is low cost since the method of making such only requires heating the low-melting metallic materials to a liquid state and, possibly, the application of a negative pressure (i.e., some level of vacuum) to promote distribution of the molten metallic materials within the array of carbon nanotubes.

0027Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.